A. c. switch contacts



March 5, 1968 L. cs.'w. KNOTT 3,

' A.C. SWITCH CONTACTS Filed Feb. 17, 1966 United States Patent Ofifice 3,372,303 A.C. SWITCH CQNTACTS Leonard George Walter Knott, Hounslow, Middlesex, England, assignor to F. Devetta (Electronics) Limited, Hounslow, Middlesex, Engiand, a British company Filed Feb. 17, 1966, Ser. No. 528,304 Claims priority, application Great Britain, Sept. 28, 1965, 41,276/65 Claims. (Cl. 317-11) This invention relates to A.C. switch contacts and comprises a device for reducing and substantially eliminating the arcing and sparking which occurs when the contacts are opened or closed.

The invention consists in a device for connection across a pair of A.C. switch contacts comprising at least one silicon controlled rectifier connectable in parallel with the contacts for carrying current in either direction, split transformer means part in series with the contacts and part in series with the S.C.R. and means for deriving a control signal for the S.C.R. from the transformer means whereby the current which would otherwise produce arcing or sparking is passed through the S.C.R. and the S.C.R. is de-triggered thereby so that it switches off when the voltage across it has zero value.

Preferably a V.D.R. is connected in parallel with the S.C.R. to absorb back EMF. voltages in excess of a specific breakdown value.

Conveniently a C.R. network is provided in the control signal circuit.

The invention will be. further described with reference to the accompanying drawing which is a circuit diagram of an arrangement according to the invention.

In the drawing, a power supply is shown connected through a pair of contacts S to a load L.

One part of a primary winding W of a transformer is provided in circuit with the load.

Across the contacts there is provided a V.D.R. in series with part W of the primary winding of the transformer.

Across the V.D.R. is a bridge rectifier with diodes D D D and D and across the bridge is connected an S.C.R.

In this way current of either direction the S.C.R. when the latter is open.

The secondary winding of the transformer has parts W and W separated by a centre tap connected to one side of the S.C.R. The outer ends of windings W W are each connected via diodes D D respectively to a C.R. network and thence to the third load of the S.C.R.

It is both convenient and of greatest practical value to begin with the contacts in the closed position; this is because the function of the device is more readily understood and greatest arcing and sparking occurs during the opening of a pair of contacts.

An alternating electric current is flowing through the pair of contacts S, and the electric load L, via the transformer winding W This current in W induces a voltage into the secondary windings W W via which, diodes D D produce a full wave rectified voltage, causing a current lg to flow into the S.C.R. gate via the RC. network shown. The S.C.R. is in the ON condition, but only a minute current can flow due to the relatively low resistance of the closed contacts. Now let the contacts open, the S.C.R. will carry the load current, which flows through both W and W windings, but as these windings are wound in opposition to each other the gate current Ig ceases to fiow leaving the S.C.R. in the avalanche state in which the load current will only cease to flow through the S.C.R. when the supply voltage, appearing across the will flow through 3,372,393 Patented Mar. 5, 1968 S.C.R. is zero value. This occurs when the alternating voltage passes from one sign of voltage to the other.

Switching off electric current at zero voltage when the electric load is inductive will involve the usual back due to the collapse of the magnetic field but because the S.C.R. opens instantly at zero voltage only the breakdown voltage of the diodes, plus that of the S.C.R. is of any consequence and as can be seen in the drawing, a V.D.R. is included to prevent voltages in excess of the specified breakdown value.

When a pair of contacts with the device fitted as shown in the drawing are open and begin to close, at a distance between the contacts where ionization occurs current fiows through W winding causing the S.C.R. to conduct instantly, since the switch on time of the S.C.R. is negligible compared to that of a pair of contacts.

The inclusion of the C.R. network in the gate circuit of the S.C.R. has the effect of speeding up the switch on time of the device when ionization occurs as the condenser C charges up. The gate current reduces to a value limited by the resistance R, sufficient to maintain the ON condition of the S.C.R. ready for the switch off function of the system.

It will be evident from the above that the S.C.R. can never carry the load current for a longer period than one half cycle of the alternating current, for it can be assumed that for any anode voltage greater than zero the S.C.R. will conduct, provided sufficient gate current is flowing, then both the gate and anode current will increase with increasing voltage producing avalanche anode current in the S.C.R. in which the gate current is no longer required to maintain S.C.R. anode conduction. Only when the S.C.R. anode voltage is reduced to zero will the S.C.R. cease to conduct, at which time the load current ceases to flow, until the contacts are again switched on.

If we now consider a pair of contacts being switched as frequently as once a second, then at an alternating frequency of supply voltage of 50 c.p.s. the S.C.R. will be on for a period of one half cyclel0 milli seconds or less, i.e. a duty one percent of the time.

It is well known that the surge characteristics of both diodes and S.C.R.s is far in excess of their average ratings. By utilising the surge characteristics described above a saving of both space and cost of up to 20 times can be achieved.

The device may be said to deal with the dynamic function of switching current and not the static.

The device is in the form of a solid-state semi-conductor module and can 'be fitted to any pair of contacts, used to interrupt alternating current; once fitted the switching system becomes synchronous with zero voltage condition, i.e. interruption can only take place at zero supply voltage, hereby avoiding the usual arcing and sparking.

Various modifications may be made within the scope of the invention.

I claim:

1. A device for connection across a pair of A.C. switch contacts comprising at least one silicon controlled rectifier connectable in parallel with the contacts for carrying current in either direction, split transformer means part in series wth the contacts and part in series with the S.C.R. and means for deriving a control signal for the S.C.R. from the transformer means whereby the current which would otherwise produce arcing or sparking is passed through the S.C.R. and the S.C.R. is de-triggered thereby so that it switches oti when the voltage across it has zero value.

2. A device as claimed in claim 1 in which the S.C.R. is connected across a bridge rectifying circuit.

3. A device as claimed in claim 1 in which the means for deriving a control signal comprise a balanced rectifier circuit in the secondary of the transformer.

4. A device as claimed in claim 3 in which the control circuit includes a C.R. network.

5. A device as claimed in claim 1 in which a V.D.R. is connected across the S.C.R. to prevent back voltages in excess of a specified breakdown value.

References Cited UNITED STATES PATENTS MILTON O. HIRSHFIELD, Primary Examiner.

10 J. D. TRAYMMELL, Assistant Examiner. 

1. A DEVICE FOR CONNECTION ACROSS A PAIR OF A.C. SWITCH CONTACTS COMPRISING AT LEAST ONE SILICON CONTROLLED RECTIFIER CONNECTABLE IN PARALLEL WITH THE CONTACTS FOR CARRYING CURRENT IN EITHER DIRECTION, SPLIT TRANSFORMER MEANS PART IN SERIES WITH THE CONTACTS AND PART IN SERIES WITH THE S.C.R. AND MEANS FOR DERIVING A CONTROL SIGNAL FOR THE S.C.R. FROM THE TRANSFORMER MEANS WHEREBY THE CURRENT WHICH WOULD OTHERWISE PRODUCE ARCING OR SPARKING IS PASSED THROUGH THE S.C.R. AND THE S.C.R. IS DE-TRIGGERED THEREBY SO THAT IT SWITCHES OFF WHEN THE VOLTAGE ACROSS IT HAS ZERO VALUE. 